Method and System for Fracture Stimulation by Cyclic Formation Settling and Displacement

ABSTRACT

The present techniques provide methods and systems for fracturing reservoirs without directly treating them. For example, an embodiment provides a method for fracturing a subterranean formation. The method includes causing a volumetric decrease in a zone proximate to the subterranean formation so as to apply a mechanical stress to the subterranean formation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/407,249, filed Oct. 27, 2010, entitled METHOD AND SYSTEM FOR FRACTURESTIMULATION, and also claims the benefit of U.S. Provisional ApplicationNo. 61/544,757, filed Oct. 7, 2011, entitled METHOD AND SYSTEM FORFRACTURE STIMULATION BY CYCLIC FORMATION SETTLING AND DISPLACEMENT. Thisapplication is also related to concurrently filed International PatentApplication, Attorney Docket No. 2010EM298-B, entitled “Method andSystem for Fracture Stimulation by Formation Displacement”.

FIELD OF THE INVENTION

Exemplary embodiments of the present techniques relate to a method andsystem for fracture stimulation of subterranean formations to enhancethe recovery of hydrocarbons. Specifically, an exemplary embodimentprovides for creating fractures and other flow paths by delamination andrubblization of formations.

BACKGROUND

This section is intended to introduce various aspects of the art thatmay be topically associated with exemplary embodiments of the presenttechniques. This discussion is believed to assist in providing aframework to facilitate a better understanding of particular aspects ofthe present techniques. Accordingly, it should be understood that thissection should be read in this light, and not necessarily as admissionsof prior art.

As hydrocarbon reservoirs that are easily harvested, such as reservoirson land or reservoirs located in shallow ocean water, are used up, otherhydrocarbon sources must be used to keep up with energy demands. Suchreservoirs may include any number of unconventional hydrocarbon sources,such as biomass, deep-water oil reservoirs, and natural gas from othersources.

One such unconventional hydrocarbon source is natural gas produced fromrocks that form unconventional gas reservoirs, including, for example,shale and coal seams. Because unconventional gas reservoirs may haveinsufficient permeability to allow significant fluid flow to a wellbore,many of such unconventional gas reservoirs are currently not consideredas practical sources of natural gas. However, natural gas has beenproduced for years from low permeability reservoirs having naturalfractures. Furthermore, a significant increase in shale gas productionhas resulted from hydraulic fracturing, which can be used to createextensive artificial fractures around wellbores. When combined withhorizontal drilling, which is often used with wells in tight gasreservoirs, the hydraulic fracturing may allow formerly unpracticalreservoirs to be commercially viable.

The fracturing process is complicated and often requires numeroushydraulic fractures in a single well and numerous wells for an economicfield development. More efficient fracturing processes may provide amore productive reservoir. In other words, a greater amount of the gas,or other hydrocarbon, trapped in a relatively non-porous reservoir, suchas a tight gas, tight sand, shale layer or even a coal seam may beharvested. Accordingly, numerous researchers have explored ways toimprove fracturing.

For example, U.S. Pat. No. 3,455,391, to Matthews, et al., discloses aprocess for horizontally fracturing subterranean earth formations. Theprocess is performed by injecting a hot fluid at high pressure, untilvertical fractures are formed and then closed due to thermal expansionof the earth formation. A fluid is then injected at a pressuresufficient to form horizontal fractures.

A similar process is disclosed in U.S. Pat. No. 3,613,785, to Closmanand Phocas. In this process a wellbore is extended into the formationand a vertical fracture is generated by pressurizing the borehole. A hotfluid is injected into the formation to heat the formation, untilthermal stressing of the formation matrix material causes the horizontalcompressive stress in the formation to exceed the vertical compressivestress at a location selected for a second well. Hydraulicallyfracturing the formation through this second well can form a horizontalfracture extending into the formation.

Other approaches have focused on relieving stress in the formation, forexample, by cavitation of the formation. For example, U.S. Pat. No.5,147,111, to Montgomery, discloses a method for cavity inducedstimulation of coal degasification wells. The method can be used forimproving the initial production of fluids, such as methane, from a coalseam. To perform the method, a well is drilled and completed into theseam. A tubing string is run into the hole and liquid carbon dioxide ispumped down the tubing while a backpressure is maintained on the wellannulus. The pumping is stopped, and the pressure is allowed to builduntil it reached a desired elevated pressure, for example, 1500 to 2000psia. The pressure is quickly released, causing the coal to fail andfragment into particles. The particles are removed to form a cavity inthe seam. The cavity can allow expansion of the coal, potentiallyleading to opening of cleats within the coal seam.

A similar concept has been described in Ukraine Patent No. 35282, whichdiscloses another method for coal degasification, but through subsurfacegasification of an underburden coal seam (a coal seam that underlies thegas-containing formation). In this process, wellbores are drilledthrough an underburden coal bed so that a gasification catalyst can beapplied. Once gasification occurs and lowers the underburden pressuredue to depletion, subsidence of the overburden (e.g., the layercontaining the gas) occurs due to gravitational loading. The subsidencecan potentially create microfractures within the overburden reservoir,thereby allowing improved gas migration to the degassing wells.

It has also been noted that vertical wells and mining processes canlower stress points on coal seams, leading to increases in theproduction of coal bed methane. For example, S. Sang, et al., “Stressrelief coalbed methane drainage by surface vertical wells in China,”International Journal of Coal Geology, Volume 82, 196-203 (2010),presents a summary of studies on improved coalbed methane production bystress relief. The paper summarizes the status of engineering practice,technology, and research related to stress relief coalbed methane (CBM)drainage using surface wells in China during the past 10 years. Commentsare provided on the theory and technical progress of this method. Inhigh gas mining areas, such as the Huainan, Huaibei and Tiefa miningareas, characterized by heavily sheared coals with relatively lowpermeability, stress relief CBM surface well drainage has beensuccessfully implemented and has broad acceptance as a CBM exploitationtechnology. The fundamental theories underpinning stress relief CBMsurface well drainage include elements relating to: (1) formation layerdeformation theory, vertical zoning and horizontal partitioning, and thechange in the stress condition in mining stopes; (2) a theory regardingan Abscission Circle in the development of mining horizontal abscissionfracture and vertical broken fracture in overlaying rocks; and (3) thetheory of stress relief inducing permeability increase in protected coalseams during mining; and the gas migration-accumulation theory of stressrelief CBM surface well drainage.

Other techniques for increasing production from coal beds, and otherreservoirs, have focused on in-situ pyrolysis of hydrocarbons in areservoir, followed by production of hydrocarbons from the reservoir.All of these techniques above have focused on the treatment of thehydrocarbon reservoir itself. Further, some techniques have taught thatrelieving a stress on a reservoir may enhance the production ofhydrocarbons, for example, by allowing cleats to open up in coal seams.

Related information may be found in S. E. Laubach, et al.,“Characteristics and origins of coal cleat: A review,” InternationalJournal of Coal Geology 35 (1998), 175-207; Ian Palmer, “Coalbed methanecompletions: A world view,” International Journal of Coal Geology 82(2010), 184-195; Jack A. Pashin, “Stratigraphy and structure of coalbedmethane reservoirs in the United States: An overview,” InternationalJournal of Coal Geology 35 (1998), 209-240; Pablo F. Sanz, et al.,“Mechanical models of fracture reactivation and slip on bedding surfacesduring folding of the asymmetric anticline at Sheep Mountain, Wyoming,”Journal of Structural Geology 30 (2008), 1177-1191; V. Palchik,“Localization of mining-induced horizontal fractures along formationlayer interfaces in overburden: field measurements and prediction,”Environ. Geol. 48 (2005), 68-80; and Stephen P. Laubach, et al.,“Differential compaction of interbedded sandstone and coal,” from:Cosgrove, J. W. and Ameen, M. S. (eds.), Forced Folds and Fractures,Geological Society of London, Special Publications, 169, 51-60 (TheGeological Society of London 2000).

SUMMARY

An embodiment of the present techniques provides a method for fracturinga hydrocarbon-bearing (HC-bearing) subterranean formation, moreparticularly by directly effecting either increasing stress and strain,or decreasing stress and strain upon or within a formation or portion ofa formation that is proximately adjacent to a HC-bearing formation thatprovides the primary hydrocarbon source for desired HC production. Thedirectly applied stress and strain (whether increased, decreased, orcycled through both effects) is applied in a method that indirectlytranslates or effects the stress and strain upon the targeted HC-bearingformation, thereby effecting structural or stratagraphic alterations,fractures, rubblization, or other desired effects that increaseseffective permeability within the HC-bearing formation to enablemovement of at least a portion of the previously flow-restrictedhydrocarbons toward a wellbore. The method includes causing a bulkvolumetric decrease in a zone or formation proximate to the subterraneanformation so as to apply or affect a resultant mechanical stress andinduced strain or deformation to the proximately adjacent HC-bearingsubterranean formation. The methods disclosed herein include at leastone step of permitting volume reduction or stress reduction upon thezone proximate so as to enable some degree of settling or other movementwithin or of the generally adjacent hydrocarbon bearing subterraneanformation to assist with enhancing the effective permeability tohydrocarbon flow within the subterranean formation.

In another embodiment, the present techniques may comprise cyclicallyincreasing and decreasing the applied stress to facilitate imparting inthe HC-bearing formation, the desired permeability change. Some methodsmay also create a formation matrix distortion hysteresis in theHC-bearing formation structure that yields improved effectivepermeability. For simplicity purposes, all such formation changes,subductions, deformations, distortion, cleaving, fracturing,rubblization, microfracturing, or other formation shape or strainchanges may be referred to generally as a volumetric “decrease” orvolumetric “increase” in bulk formation volume (or volumetric“increase,” as appropriate, such as in a cyclic operation) of both thedirectly treated formation and the indirectly affected HC-bearingformation, even when an actual volumetric decrease or increase is notactually affected, but is merely facilitated by plastic or elasticformation displacement or compression of the treatment and/or HC-bearingformations and/or compression or displacement of remote compressible orincompressible strata and/or fluid.

In another embodiment, the new methods presented herein may include Amethod for fracturing a subterranean formation, comprising: using awellbore to perform one of the steps of; (a) reducing the geomechanicalstress in a zone proximate to the subterranean formation to translate ageomechanical stress change to the subterranean formation to cause amechanical dislocation of at least a portion of the subterraneanformation and create fractures within at least a portion of thesubterranean formation; and (b) applying stress in the zone proximate tothe subterranean formation to translate a geomechanical stress change tothe subterranean formation to cause a mechanical dislocation of at leasta portion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and thereafter, using thewellbore to perform the other of step (a) and step (b). In many aspects,step (a) is performed prior to step (b), while in other applications, itmay be desirable to perform step (b) prior to step (a).

In yet another embodiment, the methods included herein may provide for amethod for fracturing a subterranean formation, comprising: using awellbore to perform one of the steps of; (a) reducing the geomechanicalstress in a zone proximate to the subterranean formation to translate ageomechanical stress change to the subterranean formation to cause amechanical dislocation of at least a portion of the subterraneanformation and create fractures within at least a portion of thesubterranean formation; and (b) applying stress in the zone proximate tothe subterranean formation to translate a geomechanical stress change tothe subterranean formation to cause a mechanical dislocation of at leasta portion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and thereafter, using thewellbore to perform the other of step (a) and step (b).

Another embodiment of the present techniques provides a method forproduction of a hydrocarbon from a reservoir. The method includesexpanding a zone below a hydrocarbon reservoir to mechanically stressthe hydrocarbon reservoir and create an arch in the hydrocarbonreservoir. A relative movement may be created across a fracture surfaceto enhance conductivity.

In yet another variation for production of a hydrocarbon, the methodsmay include a method for production of a hydrocarbon from a hydrocarbonbearing formation, comprising: cycling a contraction and expansion of azone proximate to a hydrocarbon bearing subterranean formation tomechanically stress the hydrocarbon bearing subterranean formation andcreate an arch in the hydrocarbon bearing subterranean formation; andcreating a relative movement across a fracture surface to enhanceconductivity.

A hydrocarbon production system, comprising: a hydrocarbon bearingsubterranean formation; a zone proximate to the hydrocarbon bearingsubterranean formation; a stimulation well drilled to the zone; and astimulation system configured to comprise: creating a volumetricdecrease; and reversing the volumetric decrease; and repeating thevolumetric decrease for one or more cycles.

Still another embodiment provides a hydrocarbon production system thatincludes a hydrocarbon reservoir, a zone proximate to the hydrocarbonreservoir, a stimulation well drilled to the zone, and a stimulationsystem configured to create a volumetric decrease in the zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the present techniques are better understood byreferring to the following detailed description and the attacheddrawings, in which:

FIG. 1 is a diagram of a hydraulic fracturing process;

FIG. 2 is a drawing of a local stress state for an element in ahydrocarbon bearing subterranean formation;

FIG. 3 is a drawing of a first mode of fracture formation, commonlyresulting from a standard hydraulic fracturing process;

FIG. 4 is a schematic of a well treatment process, wherein a zone belowa reservoir is subjected to a volumetric decrease, placing stress on anadjacent reservoir layer;

FIG. 5 is a block diagram of a method for stimulation of a hydrocarbonbearing subterranean formation by treating a formation outside of thereservoir;

FIG. 6A is a more detailed schematic view of a delamination fracturestimulation;

FIG. 6B is a more detailed schematic view of another delaminationfracture stimulation;

FIG. 7 is a drawing of two modes of fracture formation that mayparticipate in delamination fracture stimulation as discussed herein;

FIG. 8 is a drawing of rubblization during shearing at a fractureboundary;

FIG. 9 is a drawing of an azimuthal rotation of fracture planes within aformation that may occur as a result of cyclic treatment of theformation; and

FIG. 10A is a drawing of a delamination fracturing process illustratingthe use of a separate production well and treatment well.

FIG. 10B is a drawing of another delamination fracturing processillustrating the use of a separate production well and treatment well.

DETAILED DESCRIPTION

In the following detailed description section, the specific embodimentsof the present techniques are described in connection with exemplaryembodiments. However, to the extent that the following description isspecific to a particular embodiment or a particular use of the presenttechniques, this is intended to be for exemplary purposes only andsimply provides a description of the exemplary embodiments. Accordingly,the present techniques are not limited to the specific embodimentsdescribed below, but rather, such techniques include all alternatives,modifications, and equivalents falling within the true spirit and scopeof the appended claims.

At the outset, and for ease of reference, certain terms used in thisapplication and their meanings as used in this context are set forth. Tothe extent a term used herein is not defined below, it should be giventhe broadest definition persons in the pertinent art have given thatterm as reflected in at least one printed publication or issued patent.Further, the present techniques are not limited by the usage of theterms shown below, as all equivalents, synonyms, new developments, andterms or techniques that serve the same or a similar purpose areconsidered to be within the scope of the present claims.

“Cavitation completion” or “cavitation” is a process by which an openingmay be made in a formation. Generally, cavitation is performed bydrilling a well into a formation. The formation is then pressurized inthe vicinity of the well. The pressure is suddenly released, causing thematerial in the vicinity of the well to fragment. The fragments anddebris may then be swept to the surface through the well by circulatinga fluid through the well.

“Cleat system” is the system of naturally occurring joints that arecreated as a coal seam forms over geologic time. The cleat system allowsfor the production of natural gas if the provided permeability to thecoal seam is sufficient.

“Coal” is a solid hydrocarbon, including, but not limited to, lignite,sub-bituminous, bituminous, anthracite, peat, and the like. The coal maybe of any grade or rank. This can include, but is not limited to, lowgrade, high sulfur coal that is not suitable for use in coal-fired powergenerators due to the production of emissions having high sulfurcontent.

“Coalbed methane” (CBM) is a natural gas that is adsorbed onto thesurface of coal. CBM may be substantially comprised of methane, but mayalso include ethane, propane, and other hydrocarbons. Further, CBM mayinclude some amount of other gases, such as carbon dioxide (CO₂) andnitrogen (N₂).

A “compressor” is a machine that increases the pressure of a gas by theapplication of work (compression). Accordingly, a low pressure gas (forexample, 5 psig) may be compressed into a high-pressure gas (forexample, 1000 psig) for transmission through a pipeline, injection intoa well, or other processes.

“Directional drilling” is the intentional deviation of the wellbore fromthe path it would naturally take. In other words, directional drillingis the steering of the drill string so that it travels in a desireddirection. Directional drilling can be used for increasing the drainageof a particular well, for example, by forming deviated branch bores froma primary borehole. Directional drilling is also useful in the marineenvironment where a single offshore production platform can reachseveral hydrocarbon bearing subterranean subterranean formations orreservoirs by utilizing a plurality of deviated wells that can extend inany direction from the drilling platform. Directional drilling alsoenables horizontal drilling through a reservoir to form a horizontalwellbore. As used herein, “horizontal wellbore” represents the portionof a wellbore in a subterranean zone to be completed which issubstantially horizontal or at an angle from vertical in the range offrom about 15° to about 75°. A horizontal wellbore may have a longersection of the wellbore traversing the payzone of a reservoir, therebypermitting increases in the production rate from the well.

“Exemplary” is used exclusively herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as exemplaryis not to be construed as preferred or advantageous over otherembodiments.

A “facility” is tangible piece of physical equipment, or group ofequipment units, through which hydrocarbon fluids are either producedfrom a reservoir or injected into a reservoir. In its broadest sense,the term facility is applied to any equipment that may be present alongthe flow path between a reservoir and its delivery outlets, which arethe locations at which hydrocarbon fluids either leave the model(produced fluids) or enter the model (injected fluids). Facilities maycomprise production wells, injection wells, well tubulars, wellheadequipment, gathering lines, manifolds, pumps, compressors, separators,surface flow lines, and delivery outlets. In some instances, the term“surface facility” is used to distinguish those facilities other thanwells.

“Formation” refers to a body or section of geologic strata, structure,formation, or other subsurface solids or collected material that issufficiently distinctive and continuous with respect to other geologicstrata or other characteristics that it can be mapped, for example, byseismic techniques. A formation can be a body of geologic strata ofpredominantly one type or a combination of types, or a fraction ofstrata having substantially common set of characteristics. A formationcan contain one or more hydrocarbon-bearing subterranean formations.Note that the terms formation, hydrocarbon bearing subterraneanformation, reservoir, and interval may be used interchangeably, but maygenerally be used to denote progressively smaller subsurface regions,zones, or volumes. More specifically, a geologic formation may generallybe the largest subsurface region, a hydrocarbon reservoir orsubterranean formation may generally be a region within the geologicformation and may generally be a hydrocarbon-bearing zone (a formation,reservoir, or interval having oil, gas, heavy oil, and any combinationthereof), and an interval may generally refer to a sub-region or portionof a reservoir. A hydrocarbon-bearing zone may can be separated fromother hydrocarbon-bearing zones by zones of lower permeability such asmudstones, shales, or shale-like (highly compacted) sands. In one ormore embodiments, a hydrocarbon-bearing zone may include heavy oil inaddition to sand, clay, or other porous solids.

A “fracture” is a crack, delamination, surface breakage, separation,crushing, rubblization, or other destruction within a geologic formationor fraction of formation not related to foliation or cleavage inmetamorphic formation, along which there has been displacement ormovement relative to an adjacent portion of the formation. A fracturealong which there has been lateral displacement may be termed a fault.When walls of a fracture have moved only normal to each other, thefracture may be termed a joint. Fractures may enhance permeability ofrocks greatly by connecting pores together, and for that reason, jointsand faults may be induced mechanically in some reservoirs in order toincrease fluid flow.

“Fracturing” refers to the structural degradation of a treatmentinterval, such as a subsurface shale formation, from applied thermal ormechanical stress. Such structural degradation generally enhances thepermeability of the treatment interval to fluids and increases theaccessibility of the hydrocarbon component to such fluids. Fracturingmay also be performed by degrading rocks in treatment intervals bychemical means. “Fracture network” refers to a field or network ofinterconnecting fractures.

“Fracture gradient” refers to an equivalent fluid pressure sufficient tocreate or enhance one or more fractures in the subterranean formation.As used herein, the “fracture gradient” of a layered formation alsoencompasses a parting fluid pressure sufficient to separate one or moreadjacent bedding planes in a layered formation. It should be understoodthat a person of ordinary skill in the art could perform a simpleleak-off test on a core sample of a formation to determine the fracturegradient of a particular formation.

“Geomechanical stress” (including a change related thereto) or similarphrase, refers generally to the forces external to and/or interior to aformation acting upon or within such formation, which may define astress state, condition, or property of a formation, zone, or othergeologic strata, and/or any fluid contained therein.

“Heat source” is any system for providing heat to at least a portion ofa formation substantially by conductive or radiative heat transfer. Forexample, a heat source may include electric heaters such as an insulatedconductor, an elongated member, or a conductor disposed in a conduit.Other heating systems may include electric resistive heaters placed inwells, electrical induction heaters placed in wells, circulation of hotfluids through wells, resistively heated conductive propped fracturesemanating from wells, downhole burners, exothermic chemical reactions,and in situ combustion. A heat source may also include systems thatgenerate heat by burning a fuel external to or in a formation. Thesystems may be surface burners, downhole gas burners, flamelessdistributed combustors, and natural gas distributed combustors. In someembodiments, heat provided to or generated in one or more heat sourcesmay be supplied by other sources of energy. The other sources of energymay directly heat a formation, or the energy may be applied to atransfer medium that directly or indirectly heats the formation. Forexample, an “electrofrac heater” may use electrical conductive proppedfractures to apply heat to the formation. In an electrofrac heater, aformation is hydraulically fractured and a graphite proppant is used toprop the fractures open. An electric current may then be passed throughthe graphite proppant causing it to generate heat, which heats thesurrounding formation.

“Hydraulic fracturing” is used to create single or branching fracturesthat extend from the wellbore into reservoir formations so as tostimulate the potential for production. A fracturing fluid, typically aviscous fluid, is injected into the formation with sufficient pressureto create and extend a fracture, and a proppant is used to “prop” orhold open the created fracture after the hydraulic pressure used togenerate the fracture has been released. When pumping of the treatmentfluid is finished, the fracture “closes.” Loss of fluid to permeableformation results in a reduction in fracture width until the proppantsupports the fracture faces. The fracture may be artificially held openby injection of a proppant material. Hydraulic fractures may besubstantially horizontal in orientation, substantially vertical inorientation, or oriented along any other plane. Generally, the fracturestend to be vertical at greater depths, due to the increased mass of theoverburden. As used herein, fracturing may take place in portions of aformation outside of a hydrocarbon bearing subterranean formation inorder to enhance hydrocarbon production from the hydrocarbon bearingsubterranean formation.

“Hydrocarbon production” refers to any activity associated withextracting hydrocarbons from a well or other opening. Hydrocarbonproduction normally refers to any activity conducted in or on the wellafter the well is completed. Accordingly, hydrocarbon production orextraction includes not only primary hydrocarbon extraction but alsosecondary and tertiary production techniques, such as injection of gasor liquid for increasing drive pressure, mobilizing the hydrocarbon ortreating by, for example chemicals or hydraulic fracturing the wellboreto promote increased flow, well servicing, well logging, and other welland wellbore treatments.

“Hydrocarbons” are generally defined as molecules formed primarily ofcarbon and hydrogen atoms such as oil and natural gas. Hydrocarbons mayalso include other elements, such as, but not limited to, halogens,metallic elements, nitrogen, oxygen, and/or sulfur. Hydrocarbons may beproduced from hydrocarbon bearing subterranean formations through wellspenetrating a hydrocarbon containing formation. Hydrocarbons derivedfrom a hydrocarbon bearing subterranean formation may include, but arenot limited to, kerogen, bitumen, pyrobitumen, asphaltenes, oils,natural gas, or combinations thereof. Hydrocarbons may be located withinor adjacent to mineral matrices within the earth. Matrices may include,but are not limited to, sedimentary rock, sands, silicilytes,carbonates, diatomites, and other porous media.

A “hydraulic fracture” is a fracture at least partially propagated intoa formation, wherein the fracture is created through injection ofpressurized fluids into the formation. While the term “hydraulicfracture” is used, the techniques described herein are not limited touse in hydraulic fractures. The techniques may be suitable for use inany fractures created in any manner considered suitable by one skilledin the art. Hydraulic fractures may be substantially horizontal inorientation, substantially vertical in orientation, or oriented alongany other plane. Generally, the fractures tend to be vertical at greaterdepths, due to the increased mass of the overburden.

“Hydraulic fracturing” is a process used to create fractures that extendfrom the wellbore into formations to stimulate the potential forproduction. A fracturing fluid, typically viscous, is generally injectedinto the formation with sufficient pressure, for example, at a pressuregreater than the lithostatic pressure of the formation, to create andextend a fracture. A proppant may often be used to “prop” or hold openthe created fracture after the hydraulic pressure used to generate thefracture has been released. Parameters that may be useful forcontrolling the fracturing process include the pressure of the hydraulicfluid, the viscosity of the hydraulic fluid, the mass flow rate of thehydraulic fluid, the amount of proppant, and the like.

“Imbibition” refers to the incorporation of a fracturing fluid into afracture face by capillary action. Imbibition may result in decreases inpermeation of a formation fluid across the fracture face, and is knownto be a form of formation damage. For example, if the fracturing fluidis an aqueous fluid, imbibition may result in lower transport of organicmaterials, such as hydrocarbons, across the fracture face, resulting indecreased recovery. The decrease in hydrocarbon transport may outweighany increases in fracture surface area resulting in no net increase inrecovery, or even a decrease in recovery, after fracturing.

“In-Situ” or “insitu” refers to a state, condition, or property of ageologic formation, strata, zone, and/or fluids therein, prior tochanging or altering such state, condition, or property by an actioneffecting the formation and/or fluids therein. Changes to the insituproperties may be effected by substantially any action upon theformation, such as producing or removing fluids from a formation,injecting or introducing fluids or other materials into a formation,stimulating a formation, causing a collapse such as permitting awellbore collapse or dissolving supporting strata, removing adjacentformation or fluid, heating or cooling the formation, or other actionthat effects change in the state, condition or property of theformation. The insitu state may or may not be the virgin or originalstate of the formation, but is a relative term that may in fact merelyreference a state that exists prior to undertaking some action upon theformation.

As used herein, “material properties” represents any number of physicalconstants that reflect the behavior of a rock. Such material propertiesmay include, for example, Young's modulus (E), Poisson's Ratio( ),tensile strength, compressive strength, shear strength, creep behavior,and other properties. The material properties may be measured by anycombinations of tests, including, among others, a “Standard Test Methodfor Unconfined Compressive Strength of Intact formation Core Specimens,”ASTM D 2938-95; a “Standard Test Method for Splitting Tensile Strengthof Intact formation Core Specimens [Brazilian Method],” ASTM D 3967-95aReapproved 1992; a “Standard Test Method for Determination of the PointLoad Strength Index of Rock,” ASTM D 5731-95; “Standard Practices forPreparing formation Core Specimens and Determining Dimensional and ShapeTolerances,” ASTM D 4435-01; “Standard Test Method for Elastic Moduli ofIntact formation Core Specimens in Uniaxial Compression,” ASTM D3148-02; “Standard Test Method for Triaxial Compressive Strength ofUndrained formation Core Specimens Without Pore Pressure Measurements,”ASTM D 2664-04; “Standard Test Method for Creep of Cylindrical Softformation Specimens in Uniaxial Compressions,” ASTM D 4405-84,Reapproved 1989; “Standard Test Method for Performing Laboratory DirectShear Strength Tests of formation Specimens Under Constant NormalStress,” ASTM D 5607-95; “Method of Test for Direct Shear Strength offormation Core Specimen,” U.S. Military formation Testing Handbook,RTH-203-80, available at“http://www.wes.army.mil/SL/MTC/handbook/RT/RTH/203-80.pdf” (lastaccessed on Jun. 25, 2010); and “Standard Method of Test for MultistageTriaxial Strength of Undrained formation Core Specimens Without PorePressure Measurements,” U.S. Military formation Testing Handbook,available at http://www.wes.army.mil/SL/MTC/handbook/RT/RTH/204-80.pdf”(last accessed on Jun. 25, 2010). One of ordinary skill will recognizethat other methods of testing formation specimens may be used todetermine the physical constants used herein.

“Natural gas” refers to various compositions of raw or treatedhydrocarbon gases. Raw natural gas is primarily comprised of lighthydrocarbons such as methane, ethane, propane, butanes, pentanes,hexanes and impurities like benzene, but may also contain small amountsof non-hydrocarbon impurities, such as nitrogen, hydrogen sulfide,carbon dioxide, and traces of helium, carbonyl sulfide, variousmercaptans, or water. Treated natural gas is primarily comprised ofmethane and ethane, but may also contain small percentages of heavierhydrocarbons, such as propane, butanes, and pentanes, as well as smallpercentages of nitrogen and carbon dioxide.

“Overburden” refers to the subsurface formation overlying the formationcontaining one or more hydrocarbon-bearing zones (the reservoirs). Forexample, overburden may include rock, shale, mudstone, or wet/tightcarbonate (such as an impermeable carbonate without hydrocarbons). Anoverburden may include a hydrocarbon-containing layer that is relativelyimpermeable. In some cases, the overburden may be permeable.

“Overburden stress” refers to the load per unit area or stress overlyingan area or point of interest in the subsurface from the weight of theoverlying sediments and fluids. In one or more embodiments, the“overburden stress” is the load per unit area or stress overlying thehydrocarbon-bearing zone that is being conditioned or produced accordingto the embodiments described. In general, the magnitude of theoverburden stress may primarily depend on two factors: 1) thecomposition of the overlying sediments and fluids, and 2) the depth ofthe subsurface area or formation. Similarly, underburden refers to thesubsurface formation underneath the formation containing one or morehydrocarbon-bearing zones (reservoirs).

“Permeability” is the capacity of a formation to transmit fluids throughthe interconnected pore spaces of the rock. Permeability may be measuredusing Darcy's Law: Q=(k ΔP A)/(μL), where Q=flow rate (cm³/s),ΔP=pressure drop (atm) across a cylinder having a length L (cm) and across-sectional area A (cm²), μ=fluid viscosity (cp), and k=permeability(Darcy). The customary unit of measurement for permeability is themillidarcy. The term “relatively permeable” is defined, with respect toformations or portions thereof, as an average permeability of 10millidarcy or more (for example, 10 or 100 millidarcy). The term“relatively low permeability” is defined, with respect to formations orportions thereof, as an average permeability of less than about 10millidarcy. An impermeable layer generally has a permeability of lessthan about 0.1 millidarcy. By these definitions, shale may be consideredimpermeable, for example, ranging from about 0.1 millidarcy (100microdarcy) to as low as 0.00001 millidarcy (10 nanodarcy).

“Porosity” is defined as the ratio of the volume of pore space to thetotal bulk volume of the material expressed in percent. Although thereoften is an apparent close relationship between porosity andpermeability, because a highly porous formation may be highly permeable,there is no real relationship between the two; a formation with a highpercentage of porosity may be very impermeable because of a lack ofcommunication between the individual pores, capillary size of the porespace or the morphology of structures constituting the pore space. Forexample, the diatomite in one exemplary formation type, Belridge, hasvery high porosity, at about 60%, but the permeability is very low, forexample, less than about 0.1 millidarcy.

“Pressure” refers to a force acting on a unit area. Pressure is usuallyshown as pounds per square inch (psi). “Atmospheric pressure” refers tothe local pressure of the air. Local atmospheric pressure is assumed tobe 14.7 psia, the standard atmospheric pressure at sea level. “Absolutepressure” (psia) refers to the sum of the atmospheric pressure plus thegauge pressure (psig). “Gauge pressure” (psig) refers to the pressuremeasured by a gauge, which indicates only the pressure exceeding thelocal atmospheric pressure (a gauge pressure of 0 psig corresponds to anabsolute pressure of 14.7 psia).

As previously mentioned, a “reservoir” or “hydrocarbon reservoir” isdefined as a pay zone (for example, hydrocarbon-producing zones) thatincludes sandstone, limestone, chalk, coal, and some types of shale. Payzones can vary in thickness from less than one foot (0.3048 m) tohundreds of feet (hundreds of m). The permeability of the reservoirformation provides the potential for production.

“Reservoir properties” and “Reservoir property values” are defined asquantities representing physical attributes of rocks containingreservoir fluids. The term “reservoir properties” as used in thisapplication includes both measurable and descriptive attributes.Examples of measurable reservoir property values include impedance toP-waves, impedance to S-waves, porosity, permeability, water saturation,and fracture density. Examples of descriptive reservoir property valuesinclude facies, lithology (for example, sandstone or carbonate), andenvironment-of-deposition (EOD). Reservoir properties may be populatedinto a reservoir framework of computational cells to generate areservoir model.

A “rock physics model” relates petrophysical and production-relatedproperties of a formation formation (or its constituents) to the bulkelastic properties of the formation. Examples of petrophysical andproduction-related properties may include, but are not limited to,porosity, pore geometry, pore connectivity volume of shale or clay,estimated overburden stress or related data, pore pressure, fluid typeand content, clay content, mineralogy, temperature, and anisotropy andexamples of bulk elastic properties may include, but are not limited to,P-impedance and S-impedance. A formation physics model may providevalues that may be used as a velocity model for a seismic survey.

“Shale” is a fine-grained clastic sedimentary formation with a meangrain size of less than 0.0625 mm. Shale typically includes laminatedand fissile siltstones and claystones. These materials may be formedfrom clays, quartz, and other minerals that are found in fine-grainedrocks. Non-limiting examples of shales include Barnett, Fayetteville,and Woodford in North America. Shale has low matrix permeability, so gasproduction in commercial quantities requires fractures to providepermeability. Shale gas reservoirs may be hydraulically fractured tocreate extensive artificial fracture networks around wellbores.Horizontal drilling is often used with shale gas wells.

“Stimulated Rock Volume” (SRV) describes a relatively large formationvolume that has experienced increased permeability and associatedhydrocarbon production potential through the use of changed in-situstress (either applied or reduced stress) and strain techniques, such asbut not limited to hydraulic fracturing or other related reservoirstimulation or stressing techniques. In one potential SRV scenario, anetwork of hydraulic fractures could be in communication with fracturesthat naturally occur in the formation so that the formation volumeoutside of one specific hydraulic fracture experiences improvedreservoir properties.

“Strain” is the fractional change in dimension or volume of thedeformation induced in the material by applying stress. For mostmaterials, strain is directly proportional to the stress, and dependsupon the flexibility of the material. This relationship between strainand stress is known as Hooke's law, and is presented by the formula;=E^(˜)

“Stress” is the application of force to a material, such as a through ahydraulic fluid used to fracture a formation. Stress can be measured asforce per unit area. Thus, applying a longitudinal force f to across-sectional area S of a strength member yields a stress which isgiven by f/S.

“Substantial” when used in reference to a quantity or amount of amaterial, or a specific characteristic thereof, refers to an amount thatis sufficient to provide an effect that the material or characteristicwas intended to provide. The exact degree of deviation allowable may insome cases depend on the specific context.

The force f could be compressional, leading to longitudinallycompressing the strength member, or tensional, leading to longitudinallyextending the strength member. In the case of a strength member in aseismic section, the force will typically be tension.

“Thermal fractures” are fractures created in a formation caused byexpansion or contraction of a portion of the formation or fluids withinthe formation. The expansion or contraction may be caused by changingthe temperature of the formation or fluids within the formation. Thechange in temperature may change the pressure of fluids within theformation, resulting in the fracturing. Thermal fractures may propagateinto or form in neighboring regions significantly cooler than the heatedzone.

“Tight oil” is used to reference formations with relatively low matrixpermeability and/or porosity where liquid hydrocarbon productionpotential exists. In these formations, liquid hydrocarbon production mayalso include natural gas condensate.

“Underburden” refers to the subsurface formation below or fartherdownhole than the formation containing one or more hydrocarbon-bearingzones (the reservoirs). For example, underburden may include rock,shale, mudstone, or wet/tight carbonate (such as an impermeablecarbonate without hydrocarbons). An underburden may include ahydrocarbon-containing layer that is relatively impermeable. In somecases, the underburden may be permeable. The underburden may be aformation that is distinct from the HC-bearing formation or may be aselected fraction within a common formation shared between theunderburden portion and the HC-bearing portion. Intermediate layers mayalso reside between the underburden layer and the HC-bearing zone.

The “Young's modulus” of a formation or rock sample is the stiffness ofthe formation sample, defined as the amount of axial load (or stress)sufficient to make the formation sample undergo a unit amount ofdeformation (or strain) in the direction of load application, whendeformed within its elastic limit. The higher the Young's modulus, theharder it is to deform. It is an elastic property of the material and isusually denoted by the English alphabet E having units the same as thatof stress.

Overview

Exemplary embodiments of the present techniques provide techniques forfracture stimulation of reservoirs, or portions of a reservoir, on alarge scale, up to stimulating an entire reservoir at once. Thetechniques may be used with any type of hydrocarbon bearing subterraneanformation, such as oil, gas, or mixed reservoirs and may also be used tofracture other types of formations, such as formations used for theproduction of geothermal energy. In exemplary embodiments, thetechniques can be used to enhance production of natural gas fromunconventional (i.e., low permeability) gas reservoirs.

The stimulation is generally based on changes to formations other thanthe target formation itself, for example, by changing a volume of aproximate formation, which places a stress on the target formation. Theapplied stress can cause delamination of layers and other forms ofnon-hydraulic fracturing in the target formation, leading to theformation of cracks over a broad area. The cracks or fractures mayresult from a residual or “hysteresis” displacement of the formationcomponents due to the strain displacement that remains, both while thestress is applied and after the stress is relaxed. The hysteresis effectresults from the failure of the crack or fracture to heal completely, inthe event further fracturing happens and/or the applied stress isreduced. Thereby, the permeability may be at least somewhat permanentlyimproved. Ideally, the stress (applied initially in the zone proximateand then translated or otherwise promulgated into the hydrocarboncontaining subsurface formation) creates some residual permeability inat least a portion of the targeted subterranean formation. The treatmentduration may range from seconds, such as if explosives are used, tomonths, such as if cycling of treatments between reducing the in-situstress and increasing the stress in the zone proximate are used to openor fracture the subterranean formation rock.

At the delaminated fractures, the formation surfaces or rock stratawithin the formation can be destroyed, forming a rubble layer orinterface between the surfaces, or the formation surfaces offset fromtheir original position, forming open apertures between the surfaces. Ifthe volume changes in the proximate formation are repeated, therubblization may be increased, forming channels through which naturalgas, other hydrocarbons, or heated water, may be harvested. The use ofan applied mechanical stress may be considered counterintuitive; as suchstress would normally tend to close fractures or cleats, leading tolower production. However, in exemplary embodiments, the application ofstress may provide increased permeability and production rates, due todelamination along weak layers and rubblization within the targetreservoir, as mentioned above and discussed in further detail below.

FIG. 1 is a diagram of a hydraulic fracturing process 100. Thetraditional method of fracture stimulation utilizes “hydraulic” pressurepumping and is a proven technology that has been used since the 1940s inmore than 1 million wells in the United States to help produce oil andnatural gas. In typical oilfield operations, the technology involvespumping a water-sand mixture into subterranean layers where the oil orgas is trapped. The pressure of the water creates tiny fissures orfractures in the rock. After pumping is finished the sand props open thefractures, allowing the oil or gas to escape from the HC-bearingformation and flow to a wellbore.

For example, a well 102 may be drilled through an overburden 104 to ahydrocarbon bearing subterranean formation 106. Although the well 102may penetrate through the hydrocarbon bearing subterranean formation 106and into the underburden 108, perforations 110 in the well 102 candirect fluids to and from the hydrocarbon bearing subterranean formation106. The hydraulic fracturing process 100 may utilize an extensiveamount of equipment at the well site. This equipment may include fluidstorage tanks 112 to hold the fracturing fluid, and blenders 114 toblend the fracturing fluid with other materials, such as proppant 116and other chemical additives, forming a low pressure slurry. The lowpressure slurry 118 may be run through a treater manifold 120, which mayuse pumps 122 to adjust flow rates, pressures, and the like, creating ahigh pressure slurry 124, which can be pumped down the well 102 tofracture the rocks in the hydrocarbon bearing subterranean formation106. A mobile command center 126 may be used to control the fracturingprocess.

The goal of hydraulic fracture stimulation is to create ahighly-conductive fracture zone 128 by engineering subsurface stressconditions to induce pressure parting of the formation in thehydrocarbon bearing subterranean formation 106. This is generallyperformed by injecting fluids with a high permeability proppant 116,such as sand, into the hydrocarbon bearing subterranean formation 106 toovercome “in-situ” stresses and hydraulically-fracture the reservoirrock. The fracture zone 128 may be considered a network or “cloud” offractures generally radiating out from the well 102. Depending on thedepth of the hydrocarbon bearing subterranean formation 106, thefractures may often be predominately perpendicular to the beddingplanes, e.g., vertical within the subsurface.

After the fracturing process 100 is completed, the treating fluids areflowed back to minimize formation damage. For example, contact with thefracturing fluids may result in imbibement of the fluids by pores in thehydrocarbon bearing subterranean formation 106, which may actually lowerthe productivity of the reservoir. Further, a carefully controlledflowback may ensure proper fracture closure, trapping the proppant 116in the fractures and holding them open. Stimulation is generallyeffective at near-well scale, for example, in which the fracturedimensions are in the 100s of feet. Treating and production are oftenconducted in the same interval, e.g., the portion of the hydrocarbonbearing subterranean formation 106 reached by the well 102. Thefracturing process 100 may use significant amounts of freshwater andproppant materials. The orientation of the fractures is controlled bythe local stresses in the hydrocarbon bearing subterranean formation 106as discussed further with respect to FIG. 2.

FIG. 2 is a drawing of a local stress state 200 for an element 202 in ahydrocarbon bearing subterranean formation. The state of stress in theearth is defined by the mass of the overburden, the pressure in thepores of the rock, the tectonic stresses governing boundary conditions,and the basic mechanical properties of the rock, such as Young's modulusor stiffness. The in-situ earth stresses determine the predominantorientation of hydraulic fractures. The presence of natural fractures,the configuration of the completion itself, and the characteristics ofthe treating fluids may alter the earth stresses near the well andthereby influence growth of hydraulic fractures for a relatively shortdistance away from the well.

The earth stresses can be divided into three principal stresses whereσ_(z) is the vertical stress in this drawing, σ_(max) is the maximumhorizontal stress, while σ_(min) is the minimum horizontal stress, whereσ_(z)>σ_(max)>σ_(min). However, depending on geologic conditions, thevertical stress could be the intermediate (σ_(max)) or minimum stress(σ_(min)). These stresses are normally compressive and vary in magnitudethroughout the reservoir, particularly in the vertical direction andfrom layer to layer. The magnitude and direction of the principalstresses are important because they control the pressure required tocreate and propagate a fracture in the reservoir, the shape of thefracture, the vertical extent of the fracture, the direction of thefracture, and the stresses trying to crush or embed the propping agentduring production. Fractures in a horizontal direction, e.g.,perpendicular to a vertically drilled well or parallel to a horizontallydrilled well, may be more effective at conducting hydrocarbons back tothe well for production. However, in deeper wells, the vertical stressesmay often force fractures to be predominately vertical, e.g.,perpendicular to a horizontally drilled wellbores. As pressure on thehydrocarbon bearing subterranean formation drops, for example, duringproduction, further fracturing may be horizontal. This is discussed infurther detail with respect to FIG. 9.

In other exemplary aspects or description, the earth stresses can bedivided into three principal stresses where σ_(v) is the verticalstress, σ^(H) _(max) is the maximum horizontal stress (similar toσ_(max) in the paragraph above) and σ^(h) _(min) is the minimumhorizontal stress. Typically, these stresses are normally compressiveand vary in magnitude throughout the reservoir, particularly in thevertical direction and from layer to layer. The vertical stress σ_(v),is typically the most compressive stress, i.e., σ_(v)>σ^(H) _(max)>σ^(h)_(min). However, depending on geologic conditions, the vertical stresscould be less compressive than the maximum horizontal stress, σ^(H)_(max), or than the minimum horizontal stress, σ^(h) _(min).

Fractures in a horizontal direction, e.g., perpendicular to a verticallydrilled well or parallel to a horizontally drilled well, may be moreeffective at conducting hydrocarbons back to the well for production. Indeeper wells, the higher vertical stress from the overburden may oftenforce fractures to be predominately vertical, e.g., perpendicular to ahorizontally drilled wellbore.

FIG. 3 is a drawing of a first mode (mode I) 300 of fracture formation,commonly resulting from a standard hydraulic fracturing process.Fractures generally propagate in one or more of three primary modes asdiscussed with respect to FIGS. 3 and 7. While, each mode is capable ofpropagating a fracture, standard hydraulic fracture stimulationpredominantly utilizes mode I 300, resulting from “direct” fluidpressure parting of the rock. In mode I 300, the pressure of thehydraulic fracturing fluid either creates fractures or advancespre-existing fractures. The fractures are propagated by tensile breakingof the formation at the crack tip.

As noted herein, the fractures may often be nearly vertical andapproximately perpendicular to bedding planes. At shallow depths, thefractures produced may be horizontal, in which case they likely will beparallel to bedding planes. In standard hydraulic fracturing, thehydraulic pressure and fluids directly contact the formation beingfractured or treated. Application of the traditional hydraulicfracturing method to unconventional hydrocarbon resources, such as tightgas or shale gas reservoirs, requires both large numbers of wells andlarge numbers of fracture treatments in each well. These requirementsare largely driven by the relatively small “effective” area that iscreated during the hydraulic fracturing process due to inherentlimitations in the treating fluids, proppants, reservoir stratigraphy,and in-situ stresses. In exemplary embodiments of the presenttechniques, a new fracturing concept can be used to achieve massivefracture stimulation of wells, particularly for unconventionalhydrocarbon resources. In these embodiments, a volumetric decrease in alayer adjacent to the hydrocarbon bearing subterranean formation can beused to place a stress on the reservoir, leading to fracturing in thereservoir.

FIG. 4 is an exemplified drawing of a well treatment such as a hydraulicfracturing system 400, wherein a zone 402 below a hydrocarbon bearingsubterranean formation 404 is subjected to a volumetric contraction 406,which can place stress on the hydrocarbon bearing subterranean formation404 leading to fracturing. The techniques are not limited to ahydrocarbon bearing subterranean formation 404, but may be used in anynumber of situations where fracturing a formation layer would be useful,such as in the production of geothermal energy. In the well treatmentsystem 400, all like units are as discussed with respect to FIG. 1. Inthis exemplary embodiment, a chemical treatment may be applied in thezone 402 to create an area of cavitation. The present techniques are notlimited to a chemical treatment of the zone 402. In embodiments, thevolumetric contraction 406 may be provided through production of fluidsfrom non-hydrocarbon productive zone 402 to create subsidence in boththe non-hydrocarbon-bearing zone and in the adjacent hydrocarbon bearingsubterranean formation 404, thereby creating a network of conductivefractures in both zones, including any intermediate zones, such thathydrocarbon can flow from the HC-bearing reservoir to thenon-hydrocarbon bearing zone and finally to the wellbore. In someembodiments, the network of conductive fractures may facilitateproduction of the hydrocarbons directly from the HC-bearing zonedirectly to the wellbore or another wellbore that is separate from thewellbore used for the treatment process. In other embodiments, achemical treatment may be applied in the zone 402 to create an area ofcavitation. The present techniques are not limited to a chemicaltreatment of the zone 402. In embodiments, the volumetric contraction406 may be provided through production of fluids from non-hydrocarbonproductive zone 402 to create subsidence in both thenon-hydrocarbon-bearing zone and in the adjacent hydrocarbon bearingsubterranean formation 404, thereby creating a network of conductivefractures in both zones, including any intermediate zones, such thathydrocarbon can flow from the HC-bearing reservoir to thenon-hydrocarbon bearing zone and finally to the wellbore. In someembodiments, the network of conductive fractures may facilitateproduction of the hydrocarbons directly from the HC-bearing zonedirectly to the wellbore or another wellbore that is separate from thewellbore used for the treatment process. Further, a borehole could bedrilled in the zone 402 to induce the volumetric contraction 406. Thevolumetric contraction 406 may be enhanced by alternately injecting (forexample, hours, days, weeks, months, even years) and then producingfluid in successive cycles.

In some embodiments, the formation layers of interest are mechanicallydamaged or “delaminated,” for example, by arching, or bending flexure,of the hydrocarbon bearing subterranean formation 404. The method usedto treat the hydrocarbon bearing subterranean formation 404 would needto create the stress state to impose delamination fracturing alongpreferred layers of interest. This may occur from contracting formationsin the zone 402 from below. The delamination fractures may be createdwithout pressurizing the fracture surfaces of the hydrocarbon bearingsubterranean formation 404 with treating fluids. As stimulation fluidsdo not need to contact the surfaces of the formation, the hydrocarbonbearing subterranean formation 404 may not be damaged by imbibement ofthe treating fluids. The stimulation may be effective at reservoirscale, i.e., the fracture dimensions may be on the order of 1000s offeet. Further, the treating and the production may be conducted indifferent intervals, using the same or separate wells.

FIG. 5 is a block diagram of a method 500 for stimulation of ahydrocarbon bearing subterranean formation by treating a formationoutside of the reservoir. The method 500 begins at block 502, with thedrilling and completing of a well to the treatment interval. Thetreatment interval may be a formation under the hydrocarbon bearingsubterranean formation, as generally discussed with respect to FIG. 4.In other embodiments, the treatment interval may be beside or above thehydrocarbon bearing subterranean formation, for example, if thehydrocarbon bearing subterranean formation is in a deviated formation.At block 504, the treatment interval may be treated. For example, achemical, thermal, physical, biological, and/or other treatment may beinjected or introduced into the treatment interval. In embodiments, thetreatment may be performed by successively deflating and inflating thetreatment interval to cause rubblization of the hydrocarbon bearingsubterranean formation. In some embodiments, the treatment may beperformed by successively inflating and deflating the treatment intervalto cause rubblization of the hydrocarbon bearing subterranean formation.The treatment may be performed by reducing underburden support and/orpressure and thereafter providing an expansive force such as pressure ora heat source into the treatment interval to cause inflation of thetreatment interval such as by thermal expansion. Such deflation andinflation may be cyclically performed.

At block 506, a production well is completed to the reservoir to producehydrocarbons. The production well may be drilled after stimulation fromthe treating well, thereby reducing the potential for subsequent wellintegrity or reliability issues. In embodiments, the production well maybe the same as the treatment well, for example, by creating perforationsin the well at the interval of the hydrocarbon bearing subterraneanformation, or by drilling production wells from the treatment well. Atblock 508, hydrocarbons may be produced from the production well. Itwill be clear that the techniques described herein are not limited tothe production of hydrocarbons, but may be used in other circumstanceswhere a subterranean formation is fractured to aid in the production offluid. For example, in embodiments, the techniques may be used tofracture a hot dry formation layer for use in geothermal energyproduction. Water or other fluids may then be circulated through thefractures, collected in a production well, and returned to the surfacefor harvesting heat energy. The wells are not limited to theconformations discussed above. In embodiments, various treating, andproducing well patterns and operational schemes may be considered toconcurrently optimize reservoir stimulation, gas production, and welloperability.

FIG. 6A is a more detailed schematic view of a delamination fracturestimulation 600 showing the physics that may lead to delaminationfracturing, such as by increasing the volume of (and/or increasing thestresses within) the zone proximate 606. A well 602 may be drilledthrough a hydrocarbon bearing subterranean formation 604, and into atreatment interval or zone 606 below the hydrocarbon bearingsubterranean formation 604. The treatment interval or zone 606 does nothave to be adjacent to the hydrocarbon bearing subterranean formation604, but may have one or more intervening layers 608. These layers 608may lower the chance that a treatment fluid, if used, will leak into thehydrocarbon bearing subterranean formation 604. Further, if chemicaltreatments are used, the layers 608 may assist in fixing the tailings inplace, lowering the probability that material may migrate into thehydrocarbon bearing subterranean formation 604 or other locations.

As the treatment progresses, a volumetric contraction 610 occurs in thetreatment interval or zone 606, which pulls downwards on the layers 608,forming an arch or dome 612 in the hydrocarbon bearing subterraneanformation 604. In the embodiment shown, fluids are injected into thetreatment interval or zone 606 to dilate, subside, “arch,” and shearfracture the hydrocarbon bearing subterranean formation 604. Thedistance, or vertical distance, between the zone 606 and the hydrocarbonbearing subterranean formation 604 may control the size of the area overwhich the treatment affects the hydrocarbon bearing subterraneanformation 604. A layer that is further from the hydrocarbon bearingsubterranean formation 604 may affect a wider area, but with a lowertotal movement. For example, if a treatment of a zone 606 located around50 m under the hydrocarbon bearing subterranean formation 604 caused avertical motion of about 2 cm over a distance of about 500 m, treatmentof a zone 606 located about 100 m under the hydrocarbon bearingsubterranean formation 606, using the same contraction and/or expansionconditions, may cause a vertical motion of about 1 cm over a horizontaldistance of about 1000 m. In addition to separation distance, the choiceof the treatment zone 606 may be made on the basis of formationproperties, both in the zone 606 and in the hydrocarbon bearingsubterranean formation 604.

In addition to the properties of the formation within the zone 606, theproperties of the material in the hydrocarbon bearing subterraneanformation 604 may also influence the choice of contraction techniquesand location. For example, if the hydrocarbon bearing subterraneanformation 604 is shale, a slow contraction may not open sufficientcracks, as a ductile shale may have enough plastic deformation to resealthe cracks.

A hydrocarbon bearing subterranean formation 604 may often have weakerlayers 614, or even inherent fracture planes 616. The arching can causeshear stress in the hydrocarbon bearing subterranean formation 604,leading to sliding or breaking of the hydrocarbon bearing subterraneanformation 604 along these layers 614 and fracture planes 616, asindicated by the arrows 618, creating delamination fractures 620. Thus,the delamination fracture stimulation 600 can create a highly-conductivemulti-fracture/dual-porosity reservoir system by delaminating formationlayers, parting formation within layers, and rubblizing the formation“in-situ.” The treatment operations may also create relative movement ordisplacement between the fracture surfaces along the layers 614 andfracture planes 616 to achieve fracture conductivity, for example, bycreating delamination fractures 620 that contain enhanced permeabilityformation debris. Vertical fractures 622 may also be created during thedelamination process. The control of stresses in the formation may beused to control the direction of the fractures, as discussed withrespect to FIGS. 9 and 10.

In addition to the injection of fluids, embodiments may inducedelamination fractures in the hydrocarbon bearing subterranean formation604 by producing fluid from zone 606, to decrease the volume of thetreatment interval or zone 606 and thereby increase the stresses at thetarget formation intervals due to imposed shear stresses such thatshear-dominated fractures delaminate along, and possibly normal to, thebedding planes.

As illustrated in FIG. 6B, the methods disclosed and claimed herein alsoinclude at least one step or aspect of permitting a volume reductionand/or stress reduction upon or within the zone proximate so as toenable some responsive degree of settling or other movement within or ofthe generally adjacent hydrocarbon bearing subterranean formation toassist with enhancing the effective permeability to hydrocarbon flowwithin the subterranean formation. In some embodiments, cyclicoperations (e.g., cycling between embodiments such as illustrated inFIGS. 6A and 6B, in either order) may be utilized, whereby thesubterranean formation is, for example, expanded, displaced, orotherwise stressed to create a fracture network such as via the methodsdisclosed herein, and then allowed to shrink or move somewhat back to aninsitu volume or even beyond insitu to a further settled, distressed,and/or reduced volume (as compared to the original in-situ volume) dueto the relief from the applied stress (excepting for hysteresis volumeor permeability enhancing effects). In still other embodiments, thevolume reduction and/or stress-strain reduction may be prolonged orfurthered to effect still additional subsiding, settling, or shrinkingin volume or position is affected to cause or effect still furtherdelamination fractures in the hydrocarbon bearing subterranean formation604. Volume enhancing techniques may include using in-situ techniques,such as thermal heating, explosive detonations, and the like to enlargethe volume of the treatment interval or zone 606 and thereby increasethe stresses at the target formation intervals such that shear-dominatedfractures delaminate along, and possibly normal to, the bedding planes.Volume decreasing techniques may be cyclically followed using techniquessuch as disclosed within this discussion.

The flow conductivity of the delamination fractures may be enhanced bycyclically contracting and expanding the treatment interval or zone 606such that the delaminated formations “rubblize” due to frictionalcontact and relative sliding motion between formation surfaces, creatingan in-situ propped bed of failed formation material. This is discussedfurther with respect to FIG. 8.

In contrast with the direct hydraulic fracture stimulation of ahydrocarbon bearing subterranean formation 604, the delaminationfracture stimulation 600 minimizes direct fluid contact with theformation fracture face, thereby reducing the potential for formationdamage and the need for flowback clean-up. Further, fracture“conductivity” is created in-situ over the full fracture dimensions,thereby enhancing productivity and eliminating the need for transportingproppants. The fractures 620 may also extend beyond geologic drainageboundaries, such as faults, pinchouts and the like, reducing the numberof wells required for economic development. The fracture delamination orother permeability improvement may be created with non-aqueoustechniques to enhance volumetric strain, reducing the need forcustomized fracturing formulations and large volumes of freshwater.

In summary, the delamination fracture stimulation 600 is based on threephysical components, including delamination, rubblization, and stresscontrol. The relative importance of each of these components isdependent on the parameters of the particular application, for example,the depths of treatment interval or zone 606 and hydrocarbon bearingsubterranean formation 604, the thicknesses of each interval 604 and606, the formation properties, the pore pressures, the in-situ stressenvironments, and the like. These parameters are discussed in moredetail with respect to FIGS. 7-10.

FIG. 6B is a more detailed schematic view of a delamination fracturestimulation 601 depicting another embodiment of the physics that maylead to delamination fracturing, such as by decreasing the volume of(and/or decreasing the stresses within) the zone proximate 607. A well603 may be drilled through a hydrocarbon bearing subterranean formation605, and into a treatment interval or zone 607 below the hydrocarbonbearing subterranean formation 605. The treatment interval or zoneproximate 607 does not have to be immediately adjacent to thehydrocarbon bearing subterranean formation 605, but may be adjacent oneor more intervening layers 609. These layers 609 may lower the chancethat a treatment fluid, if used, might (potentially undesirably)leak-off, into the hydrocarbon bearing subterranean formation 605.Further, if chemical treatments are used, the layers 609 may assist infixing the tailings in place, lowering the probability that material maymigrate into the hydrocarbon bearing subterranean formation 605 or intoother undesirable locations.

As the stress or volume reducing treatment (process) progresses, avolumetric reduction 611 may occur in the treatment interval or zone607, which may exert an upward or stress increasing force outward onlayers 609, forming an inverted arch or dome 613 in the hydrocarbonbearing subterranean formation 605, either near the wellbore or at areasonable radial distance away from the wellbore. In the embodimentshown, fluids and/or formation material may be removed from thetreatment interval or zone 607 to dilate, subside, “arch,” fracture,rubblize, and/or shear at least a portion of the hydrocarbon bearingsubterranean formation 605. The distance, or vertical distance, betweenthe zone 607 and the hydrocarbon bearing subterranean formation 605 maycontrol the size of the area over which the treatment affects thehydrocarbon bearing subterranean formation 605. A layer that is furtherfrom the hydrocarbon bearing subterranean formation 605 may affect awider area, but with a lower total movement. For example, if a treatmentof a zone 607 located, say 50 m, under the hydrocarbon bearingsubterranean formation 605 caused a vertical motion of about 2 cm over adistance of about 500 m, treatment of a zone 607 located about 100 munder the hydrocarbon bearing subterranean formation 607, using the samecontraction and/or expansion conditions, may be assumed for simplifiedillustration purposes to cause a vertical motion of about 0.5 or 1 cmover a horizontal distance of about 1000 m. In addition to separationdistance, the choice of the treatment zone 607 may be made on the basisof formation properties, both in the zone 607 and in the hydrocarbonbearing subterranean formation 605.

In addition to the properties of the formation within the zone 607, theproperties of the material in the hydrocarbon bearing subterraneanformation 605 may also influence the choice of contraction techniquesand location. For example, if the hydrocarbon bearing subterraneanformation 605 is shale, a slow contraction may not open sufficientcracks, as a ductile shale may have enough plastic deformation to resealthe cracks.

A hydrocarbon bearing subterranean formation 605 may often have weakerlayers 615, or even inherent fracture planes 617. The arching or stressreduction may cause shear stress in the hydrocarbon bearing subterraneanformation 605, leading to sliding or breaking of the hydrocarbon bearingsubterranean formation 605 along these layers 615 and fracture planes617, as indicated by the arrows 619, creating delamination fractures621. Thus, the delamination fracture stimulation 601 may create ahighly-conductive multi-fracture/dual-porosity reservoir system bydelaminating formation layers, parting formation within layers, andrubbelizing the formation “in-situ.” The treatment operations may alsocreate relative movement or displacement between the fracture surfacesalong the layers 615 and fracture planes 617 to achieve fractureconductivity, for example, by creating delamination fractures 621 thatcontain enhanced permeability formation debris. Vertical fractures 623may also be created during the delamination process. The control ofstresses in the formation may be used to control the direction of thefractures, as discussed with respect to FIGS. 9 and 10.

In addition to the injection of fluids, embodiments may inducedelamination fractures in the hydrocarbon bearing subterranean formation605 by removing formation volume, fluid, and/or otherwise effectingstress reduction and formation movement from zone 607, to decrease thevolume of the treatment interval or zone 607 and thereby correspondinglydecrease or otherwise impact the stresses at the target formationintervals due to imposed shear stresses such that shear-dominatedfractures delaminate along, and possibly normal to, the bedding planes.

As illustrated in FIG. 6B, the methods disclosed and claimed hereininclude at least one step or aspect of permitting a volume reductionand/or stress reduction upon or within the zone proximate so as toenable some responsive degree of settling or other movement within or ofthe generally adjacent hydrocarbon bearing subterranean formation toassist with enhancing the effective permeability to hydrocarbon flowwithin the subterranean formation. In most embodiments, cyclicoperations (e.g., cycling between embodiments such as illustrated inFIGS. 6A and 6B, in either order) may be utilized, whereby thesubterranean formation is, for example, expanded, displaced, contracted,shrunk, collapsed, subsided, inflated, or otherwise stressed to create afracture network such as via the methods disclosed herein, and thenallowed or caused to reverse the stress change to effect an oppositeaction, such as to digress somewhat back to an insitu volume (or becaused to displace even beyond the original insitu state) to a settled,de-stressed, and/or reduced volume (as compared to the original in-situvolume) due to the relief from the applied stress (excepting forhysteresis volume or permeability enhancing effects). Cycling mayinclude a single cycle or multiple cycles, and the intervals and/ortreatment type for each cycle need not be consistent. In still otherembodiments, the volume reduction and/or stress-strain reduction may beprolonged or furthered to effect still additional subsiding, settling,or shrinking in volume or position is affected to cause or effect stillfurther delamination fractures in the hydrocarbon bearing subterraneanformation 605. Volume or stress changing techniques disclosed herein mayinclude using other in-situ techniques, such as thermal heating,explosive detonations, and steam injection, formation dissolution, etc.,to enlarge or reduce the volume or overburden supporting capacity of thetreatment interval or zone 607 and thereby increase the stresses at thetarget formation intervals such that shear-dominated fracturesdelaminate along, and possibly normal to, the bedding planes. Volumedecreasing techniques may be cyclically followed using techniques suchas disclosed within this discussion.

The flow conductivity of the delamination fractures may be enhanced bycyclically contracting and expanding the treatment interval or zone 607such that the delaminated formations “rubblize” along the fractureplanes due to frictional contact and relative sliding motion betweenformation surfaces, creating a naturally propped bed of failed formationmaterial. Such material will also create a fracture or conductivityhysteresis change due to non-reversibility of this type of destructionand displacement. This is discussed further with respect to FIG. 8.

In contrast with the conventional direct hydraulic fracture stimulationof a hydrocarbon bearing subterranean formation 605, the delaminationfracture creations 601 may minimize direct fluid contact with theformation fracture face, thereby reducing the potential for formationdamage and the need for flowback clean-up. Further, fracture“conductivity” is created in-situ over the full fracture dimensions,thereby enhancing productivity and eliminating the need for transportingproppants. The fractures 621 may also extend beyond geologic drainageboundaries, such as faults, pinchouts and the like, reducing the numberof wells required for economic development. The fracture delamination orother permeability improvement may be created with non-aqueoustechniques to enhance volumetric strain, reducing the need forcustomized fracturing formulations and large volumes of freshwater.

In summary, the delamination and/or fracture-creating treatment 601 isbased on three physical components, including delamination,rubblization, and stress control. The relative importance of each ofthese components is dependent on the parameters of the particularapplication, for example, the depths of treatment interval or zone 607and hydrocarbon bearing subterranean formation 605, the thicknesses ofeach interval 605 and 607, the formation properties, the pore pressures,the in-situ stress environments, and the like. These parameters arediscussed in more detail with respect to FIGS. 7-10.

FIG. 7 is a drawing 700 of two modes of fracture formation that mayparticipate in delamination fracture stimulation as discussed herein.Both of these modes are based on shearing the rock, rather than tensileparting of the rock. An in-plane shear mode 702 develops a fracture 704that is aligned (i.e., in the same two-dimensional plane) with theapplied shear stress 706. The in-plane shear mode 702, also termed modeII, may develop as an arch or bend that distorts a reservoir. Further,the in-plane shear mode 702 may develop horizontal fractures, forexample, as some layers 708 are placed under compressive stress, whileother layers 710 are released from compressive stress. Additional mode I300 “non-hydraulic” tensile fractures also may be incurred from stressarching of the reservoir. Another mode of fracture formation is ananti-plane shear mode 712, also termed mode III. Similarly, theanti-plane shear mode 712 develops a fracture 714 that also is alignedin the same two-dimensional plane with the applied shear stress 716.This mode may also participate in both vertical and horizontal fracturesas adjacent layers are moved in opposite directions. In embodiments,both mode II 702, and mode III 712, or any combinations thereof, maypropagate damage and fractures perpendicular or parallel to beddingplanes through the use of a volumetric decrease in layers outside of areservoir interval. The shearing modes may cause material todisaggregate.

FIG. 8 is a drawing of rubblization 800 during shearing 802 at afracture boundary 804. Direct hydraulic fracturing of a reservoirgenerally causes tensile fracturing of reservoir rocks as discussed withrespect mode I shown in FIG. 3. In contrast, the shearing 802 that takesplace in embodiments, as discussed with respect to FIG. 7, can forceformation surfaces to slide against each other at a fracture boundary804. Frictional engagement of features on the surfaces may cause theformation to break, leading to the formation of a rubblized layer withinthe fracture boundary 804.

As mentioned previously, the flow conductivity of delamination fracturesmay be enhanced by cycling the induced flexures such that thedelaminated formations “rubblize” at the fracture boundaries 804 due tofrictional contact and relative movement between formation surfaces.This process may create a propped bed of failed formation materialin-situ. Based on measurements of formation debris fields created duringmovements of faults, the thickness of the rubblized zone adjacent to thedelamination fractures may up to about 20% of the cumulative linear ortransverse movement of the fracture surfaces. Although the amount offormation debris created may be lower with each subsequent cycle,significant porosity may be created in fracture debris zones through thecyclic movement. The failed formation is referred to herein as CyclicRubblized Material (“CRM”). CRM results in secondary permeability, i.e.,dual porosity. The cycling of the induced flexures may also relievestress in the hydrocarbon bearing subterranean formation, which mayallow the fracture planes to rotate from vertical to horizontal, asdiscussed with respect to FIG. 9.

FIG. 9 is a drawing of an azimuthal rotation 900 of fracture planes 902within a formation that may occur as a result of cyclic treatment of theformation. The in-situ earth stresses determine the predominantorientation of hydraulic fractures. At shallow depths, hydraulicfractures generally are horizontal and easily create arching, uplift anddelamination fractures in formation layers above. However, at deeperdepths, hydraulic fractures generally are vertical and the horizontalstresses must be increased to locally re-orient hydraulic fractures.

As discussed above with respect to FIG. 2, the earth stresses can bedivided into three principal stresses. In this case, σ_(z) is thevertical overburden stress and is initially the highest stress in thesystem. Further, σ_(max) is the maximum horizontal stress, while σ_(min)is the minimum horizontal stress, where σ_(v)>σ_(max)>σ_(min). Although,at all depths, injection of fluids creates volumetric increases due topore dilation or formation thermal expansion, the initial fracture plane904 that forms with the treatment zone may be vertical, which may notplace an effective amount of stress on the hydrocarbon bearingsubterranean formation. Specially engineered stress conditions may shiftthe position of the overburden stress to the intermediate (σ_(max)) orminimum stress (σ_(min)), especially in regions near the well.

As a result, the axis of each successive fracture plane 902 in a cyclicfracturing process may be slightly shifted or rotated from the lastfracture plane 902, as indicated by an arrow 906. This may continueuntil a final fracture plane 908 may be horizontal. Fracturere-orientation is dependent on the characteristics of the pumpingtreatment (i.e., fluid rheology, temperature, pressure, rate, solidscontent, treatment duration, shut-down schedule), and generally occursinitially about the “azimuth” axis and subsequently about the“inclination” axis until turning horizontal.

FIG. 10A is a simplified illustration of a delamination fracturingprocess 1000 illustrating the use of a separate production well 1002 andtreatment well 1004. The techniques described herein are not limited tousing a single well for both treatment and production. In someembodiments, the treatment interval 1006 may be accessed by one or moretreatment wells 1004 other than the production well 1002 accessing thereservoir interval 1008. Furthermore, more than one treatment well 1004may be utilized to achieve a desired degree of volume increasing orstress changing stimulation treatment effect in a production well 1002.Similarly, more than one production well 1002 may be utilized for asingle treatment well 1004. Further, various combinations of treatmentwells 1004 and production wells 1002 may be located in sufficientproximity to create synergistic enhancement in their interactions.

FIG. 10B illustrates a simplified delamination fracturing process 1001indicating the use of a separate production well 1003 and treatment well1005. The techniques described herein are not limited to using a singlewell for both treatment and production. In some embodiments, thetreatment interval 1007 may be accessed by one or more treatment wells1005 other than the production well 1003 accessing the reservoirinterval 1009. Furthermore, more than one treatment well 1005 may beutilized to achieve a desired degree of volume reducing or stresschanging stimulation effect in a production well 1003. Similarly, morethan one production well 1003 may be utilized for a single treatmentwell 1005. Further, various combinations of treatment wells 1005 andproduction wells 1003 may be located in sufficient proximity to createsynergistic enhancement in their interactions.

To recap, in one embodiment, the inventive methods include a method forfracturing a subterranean formation, comprising changing the stress andstrain in a zone proximate to the subterranean formation to indirectlytranslate a mechanical stress or strain change to the subterraneanformation and effect a permeability increase within the subterraneanformation, and thereafter reversing that geomechanical stress change inthe zone proximate to at least partially reverse the fracturing orformation displacement in the subterranean formation and therebyincrease the fracturing, rubblization, and/or delamination in thesubterranean formation. The change may be created by first reducing thestress level in the zone proximate from an insitu state, and thereafterincreased to produce strain and permeability changes in the subterraneanformation; or the change may be created by first increasing the stresslevel in the zone proximate from the insitu state and thereafterdecrease the same to produce strain and permeability changes in thesubterranean formation.

A single wellbore may be used to reach both the zone proximate and thehydrocarbon bearing subterranean formation, or separate wellbore may beused for access to each of the zone proximate and the subterraneanformation. Similarly, a set of wells may be used for application of theprinciples and methods disclosed and provided herein, such as in afield-wide plan that utilizes numerous wellbores to effect thetechniques provided herein. The inventive methods and systems providedherein may also be applied using any of a variety of wellboreconfigurations, such as substantially vertical wells, horizontal wells,multi-branch wells, deviated wellbores, and combinations thereof.Similarly, the zone proximate and hydrocarbon bearing subterraneanformation may be substantially parallel or coplanar with respect to eachother, or situated in non-parallel planes, and each may comprise asingle geologic formation, zone, lens, or structure, or multipleformations, zones, lenses, or structures. The zone proximate andhydrocarbon bearing subterranean formation may also be orientedsubstantially horizontal, vertical, deviated, folded, originally arched,faulted, or irregularly positioned with respect to the wellbore and eachother.

In many embodiments, the desired permeability increase is effected bycreation of a fracture network in the subterranean formation, such as bydelamination fracturing during uplifting, down-folding or other affectedmovement of the subterranean formation. The desired permeability mayalso be the result of other types of fracturing, but is noted that forsimplification purposes, all such fracturing and displacements may bereferred to herein generally as fracturing.

The volumetric increase in the zone proximate is created by introducinga stress-inducing force into the zone proximate, such as via hydraulicfluid, explosively generated gases or pressure, thermal expansion,proppant or cuttings introduction, or other means of affecting suchforces. The introduced force may be residual and long lasting ormaintained such as via hydraulic fluid introduction, or short induration such as via explosives. Either such action may introduceresidual volume increases, even though at least a portion of the volumeincrease may be lost when the force is removed. The action in the zoneproximate is then translated or transferred into the objectiveformation, the subterranean formation, whereby a fracture orrubblization network is created within the subterranean formation.

In some embodiments, stress may be introduced into the zone proximate inthe form of, or so as to effect a reduction in, a reduction ofstructural support within the zone proximate that is then translatedinto at least a partially corresponding reduction in stability in thehydrocarbon bearing subterranean formation, resulting in creation of afracture or rubblization network within the subterranean formation.Examples of effecting a stress reduction in the zone proximate mayinclude freshwater dissolution of salt from a zone proximate, productionof water or other fluids from a zone proximate to reduce structuralsupport in the subterranean formation, chemical dissolution of the rockmaterial within the zone proximate, physical removal of portion of thezone proximate, such as via a network of relatively large or underreamedwellbores within the zone proximate, and similar actions or treatmentsto reduce structural strength of the zone proximate with respect to thein-situ, pre-treatment, or pre-action strength. In some embodiments,application and removal of the stress and strain on the zone proximatemay be cycled to cause subsequent rubblization and fracturing within thesubterranean formation.

As discussed in the above paragraphs, applying stress changes to thezone proximate may cause the zone proximate to either arch (expand, bow,collapse, settle, or otherwise displace or experience growth orreduction in volume, with the effect of such action generally being mostprominent in the vicinity of the wellbore or point of application orintroduction, and then radiating or diffusing outwardly from the pointof such application or introduction) toward or away from thesubterranean formation, whereby the subterranean formation may archcompliantly as a result of such actions in the zone proximate and astranslated through any intermediate formations. Stated differently, theapplied stress in the zone proximate produces a stress reduction orincrease in the in-situ or pre action stress level in the zoneproximate, producing strain in the zone proximate, and enables at leasta portion (the affected portion) of the subterranean formation to archtoward or away from, as appropriate, at least a portion of the zoneproximate, producing a fracture (including rubblization) network in atleast a portion of the arched or affected portion of the subterraneanformation.

In another embodiment, the methods of the present techniques may includea method for fracturing a subterranean formation, comprising: using awellbore to perform one of the steps of; (a) reducing the geomechanicalstress in a zone proximate to the subterranean formation to translate ageomechanical stress change to the subterranean formation to cause amechanical dislocation of at least a portion of the subterraneanformation and create fractures within at least a portion of thesubterranean formation; and (b) applying stress in the zone proximate tothe subterranean formation to translate a geomechanical stress change tothe subterranean formation to cause a mechanical dislocation of at leasta portion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and thereafter,performing the other of step (a) and step (b). In many embodiments, thewellbore is also used to perform the other of step (a) and step (b).

It is noted that the techniques and methods disclosed herein aredescribed generally from two different standpoints, although both areclosely related. In one standpoint, the techniques are described interms of creating or effecting “volumetric changes” (increase ordecrease, or both) in the zone proximate and/or in the subterraneanformation. As discussed herein, many of the methods used to accomplishthe objectives and techniques disclosed herein (e.g., to fracture,rubblize, delaminate a geologic formation objective to create improvedpermeability within a hydrocarbon or other reservoir or formation), mayeffect a volumetric change in such formations or zones, relative to aninsitu or pre-treatment state. From another standpoint, the methodsherein are described in terms of altering the geomechanical stresses ofa formation (including external and/or internal stresses), suchalterations including volumetric changes, but also includingdislocation, displacement, strain changes, and/or fracturing of a zoneproximate or subterranean formation, without substantial volumetricchange therein, but which otherwise none-the-less effect translation offorce, stress, and/or energy (either applied or reduced, as compared topretreatment levels) from a zone proximate to an objective subterraneanformation, The common steps include, generally, treating a zoneproximate to effect a stress change therein or thereupon, to effectpermeability increases in a hydrocarbon bearing subterranean formation.Both such descriptions are within the scope of the present inventivemethods and techniques.

As discussed herein, embodiments of the present techniques can increasewell productivity, lessen environmental impact, enhance well integrity &reliability, and improve well utilization and hydrocarbon recovery.Further, production rates and the recovery factor may be enhanced bycyclic “rubblization” over the full formation thickness. In contrast tohydraulic fracturing, which is generally halted by geological drainageboundaries, such as faults and pinchouts, delamination fractures mayextend beyond geologic drainage boundaries, thereby reducing the numberof wells and associated environmental footprint required for economicdevelopment. For example, the delamination may cover an area of aboutnine times the area of the volumetric contraction.

Still other embodiments of the claimed subject matter may include:

1. A method (500) for fracturing a subterranean formation (404, 604),comprising causing (504) a volumetric decrease (406, 610) in a zone(402, 606) proximate to the subterranean formation (404, 604) so as toapply a mechanical stress to the subterranean formation (404, 604).

2. The method of paragraph 1, wherein the zone (402, 606) is below thesubterranean formation (404, 604).

3. The method of paragraph 1, wherein the mechanical stress is appliedto only a portion of the zone (402, 604) so as to create a bendingmotion in the subterranean formation (404, 604) and cause fractures(614, 620, 622) to form through delamination (618).

4. The method of paragraph 1, further comprising:

reversing the volumetric decrease (406, 610); and

repeating the volumetric decrease (406, 610) for one or more cycles tocause rubblization (800) along a delaminated joint (804).

5. The method of paragraph 1, wherein the subterranean formation (404,604) comprises a hydrocarbon formation.

6. The method of paragraph 1, wherein creating the volumetric decrease(406, 610) comprises pumping a fluid into the zone to create a chemicalreaction (402, 604).

7. The method of paragraph 1, wherein creating the volumetric decrease(406, 610) comprises producing fluid from the zone (402, 604).

8. The method of paragraph 1, wherein creating the volumetric decrease(406, 610) comprises creating a cavitation within the zone (402, 604).

9. A hydrocarbon production system (400), comprising:

a hydrocarbon bearing subterranean formation (404);

a zone (402) proximate to the hydrocarbon bearing subterranean formation(404);

a stimulation well (102) drilled to the zone (402); and

a stimulation system configured to create a volumetric decrease (406) inthe zone (402).

10. The hydrocarbon production system of paragraph 9, wherein thehydrocarbon bearing subterranean formation (404) comprises anunconventional gas layer.

11. The hydrocarbon production system of paragraph 9, wherein the zone(402) comprises a formation layer in an underburden.

12. The hydrocarbon production system of paragraph 9, comprising aproduction well drilled into the hydrocarbon bearing subterraneanformation (404).

13. The hydrocarbon production system of paragraph 9, comprising aproduction well drilled into the hydrocarbon bearing subterraneanformation (404) from the stimulation well (102).

Still other embodiments may include the methods disclosed in thefollowing paragraphs:

1. A method for fracturing a subterranean formation, comprising:

using a wellbore to perform one of the steps of;

(a) reducing the geomechanical stress in a zone proximate to thesubterranean formation to translate a geomechanical stress change to thesubterranean formation to cause a mechanical dislocation of at least aportion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and

(b) applying stress in the zone proximate to the subterranean formationto translate a geomechanical stress change to the subterranean formationto cause a mechanical dislocation of at least a portion of thesubterranean formation and create fractures within at least a portion ofthe subterranean formation; and

thereafter, performing the other of step (a) and step (b).

2. The method of paragraph 1, wherein step (a) is performed prior tostep (b).

3. The method of paragraph 1, wherein step (b) is performed prior tostep (a).

4. The method of paragraph 1, wherein the geomechanical stress of thezone proximate in step (a) is reduced from an initial in-situgeomechanical stress state in the zone proximate to a geomechanicalstress state in the zone proximate that is less than the originalin-situ geomechanical stress of the zone proximate, prior to performingstep (b).

5. The method of paragraph 1, wherein the geomechanical stress of thezone proximate in step (a) is reduced from the applied geomechanicalstress in the zone proximate after first performing step (b).

6. The method of paragraph 5, wherein the geomechanical stress of thezone proximate in step (a) is reduced to a geomechanical stress statethat is less than the in-situ geomechanical stress of the zone proximateprior to performing step (a).

7. The method of paragraph 1, wherein the geomechanical stress of thezone proximate in step (b) is increased from an initial in-situgeomechanical stress state in the zone proximate to a geomechanicalstress state in the zone proximate that is greater than the originalin-situ geomechanical stress of the zone proximate prior to performingstep (a).

8. The method of paragraph 1, wherein the geomechanical stress of thezone proximate in step (b) is increased from the reduced geomechanicalstress in the zone proximate after first performing step (a).

9. The method of paragraph 8, wherein the geomechanical stress of thezone proximate in step (b) is increased to a geomechanical stress statethat is greater than the in-situ geomechanical stress of the zoneproximate prior to performing step (a).

10. The method of paragraph 1, wherein the geomechanical stress of thezone proximate in step (b) is increased from the reduced geomechanicalstress in the zone proximate after first performing step (a), to ageomechanical stress level in the zone proximate that is greater thanthe geomechanical stress level in the zone proximate prior to previouslyperforming step (a) in the zone proximate.

11. The method of paragraph 1, wherein the geomechanical stress of thezone proximate in step (a) is decreased from the increased geomechanicalstress in the zone proximate after first performing step (b), to ageomechanical stress level in the zone proximate that is less than thegeomechanical stress level in the zone proximate prior to previouslyperforming step (a) in the zone proximate.

12. A method for fracturing a subterranean formation, comprising:

using a wellbore to perform one of the steps of;

(a) reducing the geomechanical stress in a zone proximate to thesubterranean formation to translate a geomechanical stress change to thesubterranean formation to cause a mechanical dislocation of at least aportion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and

(b) applying stress in the zone proximate to the subterranean formationto translate a geomechanical stress change to the subterranean formationto cause a mechanical dislocation of at least a portion of thesubterranean formation and create fractures within at least a portion ofthe subterranean formation; and

thereafter, using the wellbore to perform the other of step (a) and step(b).

13. The method of paragraph 1, wherein step (a) is performed prior tostep (b).

14. The method of paragraph 1, wherein step (b) is performed prior tostep (a).

15. The method of paragraph 12, wherein the subterranean formationcomprises a hydrocarbon formation.

16. The method of paragraph 12, wherein the zone proximate comprises aformation layer in an underburden.

17. The method of paragraph 12, wherein step (a) creates a volumetricdecrease in bulk volume of the zone proximate and the volumetricdecrease is caused by a decrease in pore pressure within the zoneproximate.

18. The method of paragraph 12, wherein step (b) creates a volumetricincrease in bulk volume of the zone proximate and the volumetricincrease is caused by an increase in pore pressure within the zoneproximate.

19. The method of paragraph 17, wherein the decrease in pore pressureresults in subsidence of the subterranean formation.

20. The method of paragraph 12, wherein step (a) creates a volumetricdecrease in the zone proximate and the volumetric decrease is effectedby a method that comprises pumping a fluid into the zone proximate tocreate a chemical reaction that reduces bulk volume of the zoneproximate.

21. The method of paragraph 20, wherein the chemical reaction compriseschemicals which dissolve regions of the zone.

22. The method of paragraph 20, wherein the chemical reaction comprisesand endothermic reaction that contracts the zone.

23. The method of paragraph 12, wherein step (a) creates a volumetricdecrease in the zone proximate and the volumetric decrease is effectedproducing fluid from the zone proximate.

24. The method of paragraph 12, wherein creating the volumetric decreasecomprises material excavation from the zone proximate.

25. The method of paragraph 24, wherein the excavation within the zoneproximate comprises at least one of introduction of abrasive fluids intothe zone proximate, creating a wellbore tunnel within the zoneproximate, collapsing a wellbore within the zone proximate, creatingperforation tunnels within the zone proximate, leaching a solublematerial from the zone proximate, dissolving soluble material from thezone proximate, gasification of material from the zone proximate, anderoding formation material from the zone proximate.

26. The method of paragraph 12, further comprising producing ahydrocarbon from the subterranean formation.

27. The method of paragraph 12, further comprising producing ageothermally heated fluid from the subterranean formation.

28. A method for production of a hydrocarbon from a hydrocarbon bearingformation, comprising:

cycling a contraction and expansion of a zone proximate to a hydrocarbonbearing subterranean formation to mechanically stress the hydrocarbonbearing subterranean formation and create an arch in the hydrocarbonbearing subterranean formation; and

creating a relative movement across a fracture surface to enhanceconductivity;

29. The method of paragraph 28, wherein the hydrocarbon bearingsubterranean formation comprises a tight gas reservoir.

30. The method of paragraph 28, wherein the hydrocarbon bearingsubterranean formation comprises a shale gas reservoir.

31. The method of paragraph 28, wherein the hydrocarbon bearingsubterranean formation comprises a coal bed methane reservoir.

32. The method of paragraph 28, wherein the hydrocarbon bearingsubterranean formation comprises a tight oil reservoir.

33. The method of paragraph 28, further comprising cycling thecontraction of the zone proximate by reducing the in-situ stress in thezone proximate so as to cause at least a portion of the subterraneanformation to arch in a direction toward the zone proximate.

34. The method of paragraph 28, further comprising cycling the expansionof the zone proximate by applying stress to the zone proximate so as tocause at least a portion of the subterranean formation to arch in adirection away from the zone proximate.

35. The method of paragraph 28, wherein the relative movement across afracture surface creates a stimulated formation volume

36. The method of paragraph 34, further comprising producing ahydrocarbon from the hydrocarbon bearing subterranean formation.

37. The method of paragraph 34, comprising drilling a production wellfrom the stimulation well into the hydrocarbon bearing subterraneanformation.

38. The method of paragraph 28, further comprising drilling a productionwell into the hydrocarbon bearing subterranean formation after thetreatment is completed.

39. The method of paragraph 28, further comprising drilling a productionwell into the hydrocarbon bearing subterranean formation before thetreatment is completed.

40. The method of paragraph 28, further comprising where the cyclingcause the zone to rubblize a layer of material along a delaminationjoint with the hydrocarbon bearing subterranean formation.

41. A hydrocarbon production system, comprising:

a hydrocarbon bearing subterranean formation;

a zone proximate to the hydrocarbon bearing subterranean formation;

a stimulation well drilled to the zone; and

a stimulation system configured to comprise:

-   -   creating a volumetric decrease; and    -   reversing the volumetric decrease; and    -   repeating the volumetric decrease for one or more cycles.

42. The hydrocarbon production system of paragraph 41, wherein thehydrocarbon bearing subterranean formation comprises a tight gas layer.

43. The hydrocarbon production system of paragraph 41, wherein thehydrocarbon bearing subterranean formation comprises a shale gas layer.

44. The hydrocarbon production system of paragraph 41, wherein thehydrocarbon bearing subterranean formation comprises a coal bed methanelayer.

45. The hydrocarbon production system of paragraph 41, wherein thehydrocarbon bearing subterranean formation comprises a tight oil layer.

46. The hydrocarbon production system of paragraph 41, wherein the zonecomprises a formation layer in an underburden.

47. The hydrocarbon production system of paragraph 41, comprising aproduction well drilled into the hydrocarbon bearing subterraneanformation.

48. The hydrocarbon production system of paragraph 41, comprising aproduction well drilled into the hydrocarbon bearing subterraneanformation from the stimulation well.

49. A method for fracturing a subterranean formation, comprising:

causing a volumetric decrease in a zone proximate the subterraneanformation so as to apply a geomechanical stress change to thesubterranean formation, wherein the geomechanical stress change createsan arch-like bending movement in at least a portion of the subterraneanformation and causes fractures to form in the subterranean formation;

reversing the volumetric decrease in the zone proximate to cause avolumetric increase in the zone proximate so as to at least partiallyreverse the geomechanical stress change in the subterranean formation;and

thereafter repeating the volumetric decrease in the zone proximate tocause further fracturing in the subterranean formation.

50. The method of paragraph 49, wherein the caused fracturea and causedfurther caused fractures within the subterranean formation are causedthrough delamination of rock layers within the subterranean formationduring arching of the subterranean formation.

51. The method of paragraph 49, further comprising changing stress inthe zone proximate to cause at least a portion of the subterraneanformation to arch in a direction away from the zone proximate.

52. The method of paragraph 49, further comprising changing stress inthe zone proximate to cause at least a portion of the subterraneanformation to arch in a direction toward the zone proximate.

While the present techniques may be susceptible to various modificationsand alternative forms, the exemplary embodiments discussed above havebeen shown only by way of example. However, it should again beunderstood that the present techniques are not intended to be limited tothe particular embodiments disclosed herein. Indeed, the presenttechniques include all alternatives, modifications, and equivalentsfalling within the true spirit and scope of the appended claims.

What is claimed is:
 1. A method for fracturing a subterranean formation,comprising: using a wellbore to perform one of the steps of; (a)reducing the geomechanical stress in a zone proximate to thesubterranean formation to translate a geomechanical stress change to thesubterranean formation to cause a mechanical dislocation of at least aportion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and (b) applying stressin the zone proximate to the subterranean formation to translate ageomechanical stress change to the subterranean formation to cause amechanical dislocation of at least a portion of the subterraneanformation and create fractures within at least a portion of thesubterranean formation; and thereafter, performing the other of step (a)and step (b).
 2. The method of claim 1, wherein step (a) is performedprior to step (b).
 3. The method of claim 1, wherein step (b) isperformed prior to step (a).
 4. The method of claim 1, wherein thegeomechanical stress of the zone proximate in step (a) is reduced froman initial in-situ geomechanical stress state in the zone proximate to ageomechanical stress state in the zone proximate that is less than theoriginal in-situ geomechanical stress of the zone proximate, prior toperforming step (b).
 5. The method of claim 1, wherein the geomechanicalstress of the zone proximate in step (a) is reduced from the appliedgeomechanical stress in the zone proximate after first performing step(b).
 6. The method of claim 5, wherein the geomechanical stress of thezone proximate in step (a) is reduced to a geomechanical stress statethat is less than the in-situ geomechanical stress of the zone proximateprior to performing step (a).
 7. The method of claim 1, wherein thegeomechanical stress of the zone proximate in step (b) is increased froman initial in-situ geomechanical stress state in the zone proximate to ageomechanical stress state in the zone proximate that is greater thanthe original in-situ geomechanical stress of the zone proximate prior toperforming step (a).
 8. The method of claim 1, wherein the geomechanicalstress of the zone proximate in step (b) is increased from the reducedgeomechanical stress in the zone proximate after first performing step(a).
 9. The method of claim 8, wherein the geomechanical stress of thezone proximate in step (b) is increased to a geomechanical stress statethat is greater than the in-situ geomechanical stress of the zoneproximate prior to performing step (a).
 10. The method of claim 1,wherein the geomechanical stress of the zone proximate in step (b) isincreased from the reduced geomechanical stress in the zone proximateafter first performing step (a), to a geomechanical stress level in thezone proximate that is greater than the geomechanical stress level inthe zone proximate prior to previously performing step (a) in the zoneproximate.
 11. The method of claim 1, wherein the geomechanical stressof the zone proximate in step (a) is decreased from the increasedgeomechanical stress in the zone proximate after first performing step(b), to a geomechanical stress level in the zone proximate that is lessthan the geomechanical stress level in the zone proximate prior topreviously performing step (a) in the zone proximate.
 12. A method forfracturing a subterranean formation, comprising: using a wellbore toperform one of the steps of; (a) reducing the geomechanical stress in azone proximate to the subterranean formation to translate ageomechanical stress change to the subterranean formation to cause amechanical dislocation of at least a portion of the subterraneanformation and create fractures within at least a portion of thesubterranean formation; and (b) applying stress in the zone proximate tothe subterranean formation to translate a geomechanical stress change tothe subterranean formation to cause a mechanical dislocation of at leasta portion of the subterranean formation and create fractures within atleast a portion of the subterranean formation; and thereafter, using thewellbore to perform the other of step (a) and step (b).
 13. The methodof claim 12, wherein the subterranean formation comprises a hydrocarbonformation.
 14. The method of claim 12, wherein the zone proximatecomprises a formation layer in an underburden.
 15. The method of claim12, wherein step (a) creates a volumetric decrease in bulk volume of thezone proximate and the volumetric decrease is caused by a decrease inpore pressure within the zone proximate.
 16. The method of claim 12,wherein step (b) creates a volumetric increase in bulk volume of thezone proximate and the volumetric increase is caused by an increase inpore pressure within the zone proximate.
 17. The method of claim 15,wherein the decrease in pore pressure results in subsidence of thesubterranean formation.
 18. The method of claim 12, wherein step (a)creates a volumetric decrease in the zone proximate and the volumetricdecrease is effected by a method that comprises pumping a fluid into thezone proximate to create a chemical reaction that reduces bulk volume ofthe zone proximate.
 19. The method of claim 18, wherein the chemicalreaction comprises chemicals which dissolve regions of the zone.
 20. Themethod of claim 18, wherein the chemical reaction comprises andendothermic reaction that contracts the zone.
 21. The method of claim12, wherein step (a) creates a volumetric decrease in the zone proximateand the volumetric decrease is effected producing fluid from the zoneproximate.
 22. The method of claim 12, wherein creating the volumetricdecrease comprises material excavation from the zone proximate.
 23. Themethod of claim 22, wherein the excavation within the zone proximatecomprises at least one of introduction of abrasive fluids into the zoneproximate, creating a wellbore tunnel within the zone proximate,collapsing a wellbore within the zone proximate, creating perforationtunnels within the zone proximate, leaching a soluble material from thezone proximate, dissolving soluble material from the zone proximate,gasification of material from the zone proximate, and eroding formationmaterial from the zone proximate.
 24. The method of claim 12, furthercomprising producing a hydrocarbon from the subterranean formation. 25.The method of claim 12, further comprising producing a geothermallyheated fluid from the subterranean formation.
 26. A method forproduction of a hydrocarbon from a hydrocarbon bearing formation,comprising: cycling a contraction and expansion of a zone proximate to ahydrocarbon bearing subterranean formation to mechanically stress thehydrocarbon bearing subterranean formation and create an arch in thehydrocarbon bearing subterranean formation; and creating a relativemovement across a fracture surface to enhance conductivity;
 27. Themethod of claim 26, wherein the hydrocarbon bearing subterraneanformation comprises a tight gas reservoir.
 28. The method of claim 26,wherein the hydrocarbon bearing subterranean formation comprises a shalegas reservoir.
 29. The method of claim 26, wherein the hydrocarbonbearing subterranean formation comprises a coal bed methane reservoir.30. The method of claim 26, wherein the hydrocarbon bearing subterraneanformation comprises a tight oil reservoir.
 31. The method of claim 26,further comprising cycling the contraction of the zone proximate byreducing the in-situ stress in the zone proximate so as to cause atleast a portion of the subterranean formation to arch in a directiontoward the zone proximate.
 32. The method of claim 26, furthercomprising cycling the expansion of the zone proximate by applyingstress to the zone proximate so as to cause at least a portion of thesubterranean formation to arch in a direction away from the zoneproximate.
 33. The method of claim 26, wherein the relative movementacross a fracture surface creates a stimulated formation volume
 34. Themethod of claim 32, further comprising producing a hydrocarbon from thehydrocarbon bearing subterranean formation.
 35. The method of claim 32,comprising drilling a production well from the stimulation well into thehydrocarbon bearing subterranean formation.
 36. The method of claim 26,further comprising drilling a production well into the hydrocarbonbearing subterranean formation after the treatment is completed.
 37. Themethod of claim 26, further comprising drilling a production well intothe hydrocarbon bearing subterranean formation before the treatment iscompleted.
 38. The method of claim 26, further comprising where thecycling cause the zone to rubblize a layer of material along adelamination joint with the hydrocarbon bearing subterranean formation.39. A hydrocarbon production system, comprising: a hydrocarbon bearingsubterranean formation; a zone proximate to the hydrocarbon bearingsubterranean formation; a stimulation well drilled to the zone; and astimulation system configured to comprise: creating a volumetricdecrease; and reversing the volumetric decrease; and repeating thevolumetric decrease for one or more cycles.
 40. The hydrocarbonproduction system of claim 39, wherein the hydrocarbon bearingsubterranean formation comprises a tight gas layer.
 41. The hydrocarbonproduction system of claim 39, wherein the hydrocarbon bearingsubterranean formation comprises a shale gas layer.
 42. The hydrocarbonproduction system of claim 39, wherein the hydrocarbon bearingsubterranean formation comprises a coal bed methane layer.
 43. Thehydrocarbon production system of claim 39, wherein the hydrocarbonbearing subterranean formation comprises a tight oil layer.
 44. Thehydrocarbon production system of claim 39, wherein the zone comprises aformation layer in an underburden.
 45. The hydrocarbon production systemof claim 39, comprising a production well drilled into the hydrocarbonbearing subterranean formation.
 46. The hydrocarbon production system ofclaim 39, comprising a production well drilled into the hydrocarbonbearing subterranean formation from the stimulation well.
 47. A methodfor fracturing a subterranean formation, comprising: causing avolumetric decrease in a zone proximate the subterranean formation so asto apply a geomechanical stress change to the subterranean formation,wherein the geomechanical stress change creates an arch-like bendingmovement in at least a portion of the subterranean formation and causesfractures to form in the subterranean formation; reversing thevolumetric decrease in the zone proximate to cause a volumetric increasein the zone proximate so as to at least partially reverse thegeomechanical stress change in the subterranean formation; andthereafter repeating the volumetric decrease in the zone proximate tocause further fracturing in the subterranean formation.
 48. The methodof claim 47, wherein the caused fractures within the subterraneanformation are caused through delamination of rock layers within thesubterranean formation during arching of the subterranean formation. 49.The method of claim 47, further comprising changing stress in the zoneproximate to cause at least a portion of the subterranean formation toarch in a direction away from the zone proximate.
 50. The method ofclaim 47, further comprising changing stress in the zone proximate tocause at least a portion of the subterranean formation to arch in adirection toward the zone proximate.